Heat transfer tube

ABSTRACT

The present invention relates to a heat transfer pipe comprising a pipe element and a number of ribs arranged around the outer circumference of the pipe element and extending outwards, annular segments being formed encompassing the ribs on the pipe element and fastened to one another with a spring-elastic clamping device, in particular on two annular segments respectively forming a half shell.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM TO PRIORITY

This application is a national phase application of PCT/EP/2010/059673,filed on Jul. 6, 2010 the disclosure of which is incorporated herein byreference and to which priority is claimed.

DESCRIPTION Heat Transfer Pipe

The present invention relates to a latent heat accumulator or thermochemical comprising at least one heat transfer pipe comprising a pipeelement and a number of ribs arranged around the outer circumference ofthe pipe element and extending outwards and a storage materialsurrounding said heat transfer pipe.

Various heat accumulators are currently known which differ from oneanother with regard to their construction and their mode of operation.Sensitive heat accumulators use the tangible heat or the heat capacityof a material. They change the temperature level of the storage materialwhen loading and unloading. Latent heat accumulators use, for example,the enthalpy of reversible thermodynamic state changes of a storagemedium, in particular the phase transition from solid to liquid and viceversa. When loading the heat accumulator a phase change materialprovided in the latter is melted. During the melting process the phasechange material absorbs a large amount of heat energy in the form of thefusion heat. Due to the reversibility of this process the phase changematerial gives off this amount of heat again when solidifying. A similarprinciple is followed by so-called the thermo chemical heat accumulatorswhich use the enthalpy of reversible chemical reactions, such as forexample absorption and desorption processes based upon chemisorption.

When loading a heat accumulator heat must be transferred to the storagemedium. For this purpose so-called heat transfer pipes can be used bymeans of which heat is transferred indirectly from a medium flowingthrough to the storage medium. Heat transfer pipes comprise a pipeelement for guiding the medium through. In order to increase the heattransfer surface outwardly extending ribs are normally arranged aroundthe outer circumference of the pipe element. The ribs can be formedintegrally with the pipe element, they can be in metallic contact withthe pipe element, or they are connected metallically to the pipe elementby a solder or weld connection.

With the integral design the ribs are produced around the outercircumference of the pipe element for example with the aid of a rollingprocess. However, this integral design is disadvantageous in that thepipe element and the ribs must be made of the same material, due towhich the requirements for high temperature stability on the inside andhigh heat conductivity on the outside can not be fulfilled optimally.Bimetal pipes do not have this advantage, but due to the different heatexpansions of the components can only be used within a restrictedtemperature range. In this case the pipe element and rib materials canaccordingly not be chosen arbitrarily. Moreover, the configurationpossibilities for the rib geometry are restricted by the productionprocess. Therefore in particular the rib diameter that can be producedby a rolling process is too small.

As an alternative to the integral configuration, the ribs can befastened onto the outer circumference of the pipe element in the form ofseparate components. For example it is known to provide annular elementsin the form of metal sheets defining ribs that are arranged around theouter circumference of the pipe element, whereupon the pipe can bewidened from the inside in order to secure the annular element on thepipe element. However, a poor thermotechnical connection of the pipeelement and ribs is achieved here. The maximum temperature for use isalso restricted. Moreover, the wall thickness of the pipe element islimited in that it must be allowed to widen to the desired degree.

Furthermore, annular elements of the type described above or also ribsin the form of strips of metal sheet or the like can be soldered orwelded to the outer circumference of the pipe element. The compatibilityof the solder must be guaranteed here however.

DE-A-2002572 discloses an environmental air evaporator with heattransfer pipes, wherein the ribs are formed as annular segmentsencompassing the pipe elements in a clamping manner.

U.S. Pat. No. 3,280,907 discloses a heat transfer device having a heattransfer pipe, which is designed in a similar way, wherein the annularsegments forming the ribs can be held on the corresponding pipe elementby means of a spring-elastic clamping device.

Depending on the production method the currently available heat transferpipes differ therefore as regards the maximum temperature for use, thepossible configurations as regards the rib geometry, the maximum wallthickness of the pipe element, the quality of the thermotechnicalconnection between the pipe element and the ribs, the compatibility ofthe materials to the pipe element and the ribs, and the compatibility ofthe solder and weld materials.

Proceeding from this prior art it is an object of the present inventionto provide a latent heat accumulator or thermo chemical accumulatorinitially mentioned kind wherein the configuration of the rib geometry,in particular of the rib diameter, the maximum temperature for use, themaximum wall thickness of the pipe element as well as the materials ofthe pipe and of the ribs can be chosen arbitrarily, wherein a goodthermotechnical connection between the pipe element and the ribs is tobe produced, and wherein a proper function is ensured.

In order to achieve this object the present invention provides a heattransfer pipe of the initially mentioned kind, wherein the ribs areformed on annular segments encompassing the pipe element and fastened toone another with a spring-elastic clamping device, in particular on twoannular segments respectively forming a half shell and wherein the atleast one heat transfer pipe is arranged substantially vertically withinthe heat accumulator. The provision of a number of annular segmentsencompassing the pipe element and fastened to one another with aspring-elastic clamping device makes it possible to use materials withdifferent expansion coefficients for the pipe element and to use theribs at high maximum temperatures of use. Due to their spring elasticitythe clamping device always guarantees good thermal contact between thepipe element and the annular segments, and accordingly a goodthermotechnical connection between the pipe element and the ribs becauseit reacts flexibly to thermal material expansions. With the design ofthe clamping device in the form of at least one spring-elastic clampelement extending in the longitudinal direction of the heat transferpipe the thermal contact between the pipe element and the annularsegments is moreover constant over the length of the annular segments.Furthermore, the configuration of the ribs, in particular the ribdiameter, can be chosen substantially arbitrarily.

The substantially vertical arrangement of the at least one heat transferpipe within the heat accumulator ensures free volume expansion of thephase change material in the vertical direction within the heataccumulator during the phase change from solid to liquid or vice versa,whereby a proper operation of the latent heat accumulator is guaranteed.The same applies for the thermo chemical accumulator.

The maximum wall thickness of the pipe element is in no way restrictedeither. According to one configuration of the present invention theclamp element(s) is/are formed from spring steel. Accordingly, thermallycaused expansions or shrinkage can be flexibly absorbed so that a verygood thermotechnical connection of the pipe element and the annularsegments is always guaranteed.

According to one configuration of the present invention each clampelement is a flexible part comprising a base section and two clampsections to be extended over one another from opposite sides of the basesection, end sections preferably splayed apart from one anotheradjoining the clamp sections and acting as a push-on aid, for examplewhile pushing onto a clamping bar formed on the annular segments.Accordingly each clamp element has a substantially omega-shapedcross-section.

The annular segments are preferably in the form of extrusion mouldedprofiles, the ribs extending in the longitudinal direction of the pipeelement. The extrusion moulding constitutes on the one hand aninexpensive production method. On the other hand the cross-section ofthe ribs is constant in the longitudinal direction of the heat transferpipe, and when used within a heat accumulator this allows free volumeexpansion of the storage material in the longitudinal direction of theheat transfer pipe. Accordingly, thermomechanical stresses caused by thevolume expansion of the storage material can be eliminated or at leastreduced. This is particularly significant with latent heat accumulatorsbecause the phase change materials used in the latter undergo a largevolume expansion in the phase change from solid to liquid or vice versa.

Preferably the geometry of the pipe element and that of the annularsegments are matched to one another such that the annular segments restwith substantially their entire surface against the pipe element. Inthis way good heat transfer from the pipe element to the annularsegments is guaranteed.

In order to increase the rib surface area the latter advantageously havebranches.

According to one configuration of the present invention the annularsegments have clamping bars which are encompassed by the clamp elements.The clamping bars are form-matched to the clamp elements here so thatthe clamp elements are easy to fit and a secure hold of the annularsegments against the pipe element with good contacting is guaranteed.

The annular segments are preferably produced from an aluminium alloy orfrom unalloyed aluminium, in particular from alloys of aluminium groups1xxx, 3xxx and 6xxx. These materials have shown to be particularlyadvantageous.

The pipe element can be a welded or a drawn pipe element.

The pipe element is preferably produced from steel or stainless steel.

In a latent heat accumulator, the storage material surrounding the heattransfer pipe is a phase change material, particularly salts or saltmixtures, in particular alkali metal nitrates, such as for example NaNO3or KNo₃-NaNO₃, nitrites, sulphates, carbonates, chlorides, hydroxides,bromides, thiocyanates and fluorides or combinations of the latter, inparticular anhydrous salts with a melting temperature of above 120° C.or salt hydrates with a maximum phase transformation up to 200° C.

Further features and advantages of the present invention are describedbelow using one embodiment of a heat transfer pipe according to theinvention with reference to the attached drawings. These show asfollows:

FIG. 1 a perspective view that shows a heat transfer pipe according toone embodiment of the present invention;

FIG. 2 a side view of the heat transfer pipe shown in FIG. 1 which showsthe face side of the latter; and

FIG. 3 a diagram that shows the temperature developments over aplurality of melting/solidifying cycles.

KEY TO FIG. 3

Elektrische Leistunq vom luftgekühlten Stab [W]=electrical power of theair-cooled bar [W]Zeit [min]=time [mins]

FIGS. 1 and 2 show a heat transfer pipe 10 according to one embodimentof the present invention. The heat transfer pipe 10 comprises a pipeelement 12, two annular segments 16 and 18 forming ribs 14, 15 and whichrespectively form a half shell disposed around the outer circumferenceof the pipe element 12 and surrounding the latter, and two clampelements 20 with which the annular segments 16 and 18 are fastened toone another and held on the pipe element 12.

The pipe element 12 is a steel pipe with a circular lateral surface asviewed in cross-section. The annular segments 16 and 18 are respectivelyin the form of extrusion moulded profiles made of an aluminium alloy sothat they have a constant cross-section in the longitudinal direction L.The geometry of the pipe element 12 and that of the annular segments 16and 18 are matched to one another so that the annular segments 16 and 18respectively lie with substantially their entire surface with an annularsegment section 22 and 23 having a circular segment-shaped cross-sectionagainst the lateral surface of the pipe element 12. On the free ends ofthe annular segment sections 22, 23 respectively radially outwardlyprojecting clamping bars 24, 25 are formed which are encompassed by theclamp elements 20. In order to guarantee a secure seat of the clampelements 20 the clamping bars 24, 25 are respectively provided on theirfree ends with snap tabs 26, 27. The ribs 14, 15 of the annular segments16 and 18 have a plurality of branches by means of which the surfacearea of the ribs 14, 15 is increased.

The clamp elements 20 are flexible parts produced from spring steelcomprising a base section 28 and two clamp sections 30 and 32 to beextended over one another from opposite sides of the base section 28,against which end sections 34 and 36 splayed apart from one another andwhich serve as a push-on aid rest.

In order to fit the heat transfer pipe 10 shown in FIGS. 1 and 2 theannular segments 16 and 18 are laid with their annular segment sections22, 23 around the lateral surface of the pipe element 12. The four clampelements 20 are then pushed onto the clamping bars 24, 25 formed on theannular segments 16 and 18 so that the annular segments 16 and 18 arefastened to one another and held by pressure against the pipe element12. In this state the annular segment sections 22, 23 of the annularsegments 16 and 18 lie with substantially their entire surface againstthe lateral surface of the pipe element 12.

The configuration of the heat transfer pipe 10 shown in FIGS. 1 and 2 isadvantageous in that by providing a number of annular segments 16 and 18surrounding the pipe element 12 and fastened to one another with clampelements 20, the use of materials with different expansion coefficientsfor the pipe element 12 and the ribs 14, 15 at high maximum temperaturesof use is made possible, such as in the present case steel for the pipeelement 12 and an aluminium alloy for the ribs 14, 15. Moreover, theclamp elements 20 always guarantee a good thermal contact between thepipe element 12 and the annular segments 16 and 18, and accordingly agood thermotechnical connection between the pipe element 12 and the ribs14, 15 because they can react flexibly to thermal material expansions.Furthermore, the configuration of the ribs 14, 15 can substantially bechosen arbitrarily, and this applies in particular to the outer diameterof the ribs 14, 15. The maximum wall thickness of the pipe element 12 isin no way restricted either.

FIG. 3 shows the results of a series of tests which was carried out witha heat transfer pipe 10 of the type shown in FIGS. 1 and 2. For thispurpose the heat transfer pipe 10 was inserted into a latent heataccumulator (not detailed). 1,070 g sodium nitrate with a meltingtemperature of 306° C. was used as a phase change material. The heat wasintroduced and removed via the heat transfer pipe 10 by means of aircooling and electrical heating. The quality of the connection producedwith the aid of the clamp elements 20 between the pipe element 12 andthe annular segments 16 and 18 was determined over 115melting/solidifying cycles. One cycle was sub-divided here into fourconsecutive phases as follows:

1) isothermal heating at 280° C. for 3 hours;2) implementation of a temperature jump from 280° C. to 330° C.;3) isothermal heating at 330° C. for 3 hours; and4) implementation of a temperature jump from 330° C. to 280° C.

The temperature progressions T₁, T₁₀, T₃₀, T₅₀ and T₁₁₅ (the indexcorresponds to the cycle number) in FIG. 3 show that the time untilthere is total introduction of the heat with a jump from 280° C. to 330°C. at the time “5 minutes” (phases 2 and 3) is kept almost constant overthe number of cycles. After approx. 35 minutes there is a state ofequilibrium with all measurements. In this state the flow of heat on thelateral surface of the pipe element 12 is almost 0. Therefore theresults show that the contact resistance between the pipe element 12 andthe annular segments 16 and 18 is practically unchanged over 115melting/solidifying cycles.

1. Latent heat accumulator or thermo chemical accumulator comprising atleast one heat transfer pipe with a pipe element and a number of ribsarranged around the outer circumference of the pipe element andextending outwards and a storage material surrounding said heat transferpipe, wherein the ribs are formed on annular segments encompassing thepipe element and fastened to one another with a spring-elastic clampingdevice, in particular on two annular segments respectively forming ahalf shell, wherein the at least one heat transfer pipe is arrangedsubstantially vertically within the heat accumulator.
 2. Latent heataccumulator or thermo chemical accumulator according to claim 1, whereinthe clamping device has at least one spring-elastic clamp element. 3.Latent heat accumulator or thermo chemical accumulator according toclaim 2, wherein the clamp elements are produced from spring steel. 4.Latent heat accumulator or thermo chemical accumulator according toclaim 2, wherein each clamp element is a flexible part comprising a basesection and two clamp sections to be extended over one another fromopposite sides of the base section end sections preferably splayed apartfrom one another adjoining the clamp sections and acting as a push-onaid.
 5. Latent heat accumulator or thermo chemical accumulator accordingto claim 1, wherein the annular segments are in the form of extrusionmolded profiles, the ribs extending in the longitudinal direction of thepipe element.
 6. Latent heat accumulator or thermo chemical accumulatoraccording to claim 1, wherein the geometry of the pipe element and thatof the annular segments are matched to one another such that the annularsegments rest with substantially their entire surface against the pipeelement.
 7. Latent heat accumulator or thermo chemical accumulatoraccording to according to claim 1, wherein the ribs have branches. 8.Latent heat accumulator or thermo chemical accumulator according toclaim 2, wherein the annular segments have clamping bars which areencompassed by the clamp elements.
 9. Latent heat accumulator or thermochemical accumulator according to claim 1, wherein the annular segmentsare produced from an aluminium alloy or from unalloyed aluminium, inparticular from alloys of aluminium groups 1xxx, 3xxx and 6xxx. 10.Latent heat accumulator or thermo chemical accumulator according toclaim 1, wherein the pipe element is a welded or a drawn pipe element.11. Latent heat accumulator or thermo chemical accumulator according toclaim 1, wherein the pipe element is produced from steel or fromstainless steel.
 12. (canceled)
 13. Latent heat accumulator or thermochemical accumulator according to claim 1, wherein the storage materialsurrounding the heat transfer pipe is a phase change material, inparticular in the form of salts and salt mixtures, in particular alkalimetal nitrates, nitrites, sulphates, carbonates, chlorides, hydroxides,bromides, thiocyanates and fluorides or combinations of the latter, inparticular anhydrous salts with a melting temperature above 120° C. orsalt hydrates with a maximum phase transformation temperature of up to200° C.
 14. (canceled)
 15. Latent heat accumulator or thermo chemicalaccumulator according to claim 3, wherein each clamp element is aflexible part comprising a base section and two clamp sections to beextended over one another from opposite sides of the base section, endsections preferably splayed apart from one another adjoining the clampsections and acting as a push-on aid.
 16. Latent heat accumulator orthermo chemical accumulator according to claim 4, wherein the annularsegments are in the form of extrusion molded profiles, the ribsextending in the longitudinal direction of the pipe element.
 17. Latentheat accumulator or thermo chemical accumulator according to claim 5,wherein the geometry of the pipe element and that of the annularsegments are matched to one another such that the annular segments restwith substantially their entire surface against the pipe element. 18.Latent heat accumulator or thermo chemical accumulator according toaccording to claim 5, wherein the ribs have branches.
 19. Latent heataccumulator or thermo chemical accumulator according to claim 5, whereinthe annular segments have clamping bars which are encompassed by theclamp elements.
 20. Latent heat accumulator or thermo chemicalaccumulator according to claim 6, wherein the annular segments haveclamping bars which are encompassed by the clamp elements.
 21. Latentheat accumulator or thermo chemical accumulator according to claim 9,wherein the pipe element is a welded or a drawn pipe element.
 22. Latentheat accumulator or thermo chemical accumulator according to claim 2,wherein the storage material surrounding the heat transfer pipecomprises anhydrous salts with a melting temperature above 120° C. orsalt hydrates with a maximum phase transformation temperature of up to200° C.